A. Field of the Invention
This invention relates to a thin film transistor and, more particularly, to an improvement of an electrode structure of a thin film transistor (hereinafter referred to as a "TFT"), and the process of making the TFT.
Thin film transistors are advantageous in many circumstances. For example, when TFTs are added to a liquid crystal display unit having electrodes arranged in matrix form or to an electroluminescence panel, the display capacity is increased thereby improving the quality of the display, and the peripheral drive circuitry can be simplified. Thin film transistors have not been put in practical use as yet because the manufacturing process for TFTs is technically difficult and TFTs are still not sufficiently reliable and stable.
A TFT has a thin film structure in which a source electrode and a drain electrode are formed on a semiconductor film layer, and an insulating film is formed if necessary. The required characteristics of source and drain electrode materials are as follows:
(1) The source and drain electrode materials should be in ohmic contact with the semiconductor film. This condition is essential for the source and drain electrode materials of the transistors. If ohmic contact is not achieved a voltage drop occurs at the contact region between the source and drain electrodes and the semiconductor, and, the output voltage is decreased. To prevent a decrease in the output voltage it is necessary to increase the driving voltage by an amount equal to the voltage drop across the contact region. When the driving voltage is increased the gate voltage increases proportionally and the reliability of the TFT is lowered.
(2) The electrode materials should be thin film materials which adhere to the semiconductor and the substrate. If the adhesion force of the source and drain electrode film is weak, the film may come off either during or after formation of the film or during source and drain electrode pattern removal. The adhesion strength of such a film can be increased by various pretreatment methods, for example by cleaning the surface of the substrate by exposing it to plasma discharge or by a variety of film forming methods, for example, forming the film by ion plating. The most commonly employed method is to increase the temperature of the substrate during formation of the film. This method is advantageous because increasing the temperature of the substrate removes any gas and/or moisture from the surface of the substrate., thereby, cleaning it, and accelerating the chemical binding of the film material to the substrate. The temperature to which the substrate can be heated is limited by the type of semiconductor film and the source and drain electrode pattern forming method. The temperature limitation is applicable when the semiconductor is a material such as tellurium (Te) or cadmium sulfide (CdS) having a relatively high vapor pressure and when the source and drain electrode pattern forming method is a lift-off method using a resist mask. In general, the substrate should be heated to 200.degree. C. to 250.degree. C. or higher in order to increase the force of adhesion. However,if a semiconductor material which has a high vapor pressure is heated to a certain temperature the semiconductor film will evaporate. If the substrate temperature is too high when using resist masks and the lift-off method, the amount of gas discharged from the resist is increased causing an adverse affect on the film and resulting in a resist seizure phenomenon. This makes it impossible to form an electrode pattern. Accordingly, it is necessary to use a thin film material which has sufficient adhesion when the substrate is at relatively low temperature during the formation of the film (for example about 100.degree. C.).
(3) The electrode materials should be readily available, low in cost, and such that they can be formed into films by ordinary sputtering or vacuum deposition methods. These characteristics are essential for reducing the manufacturing cost.
The source and drain electrode materials which meet the above-described three conditions are limited. For example, if tellurium (Te) is selected as the abovedescribed semiconductor film having a high vapor pressure, the source and drain electrodes preferably would be gold (Au), nickel (Ni), cobalt (Co) or indium (In) to have an ohmic contact with the tellurium (Te) semiconductor film. However, it has been demostrated through experiments that of these materials, gold and nickel provide the best results. Gold is expensive. Therefore, when the semiconductor film is made of tellurium (Te), nickel (Ni) satisfies the above-described conditions. B. Description of the Prior Art
A conventional method of manufacturing a thin film transistor having a semiconductor film of Te and a source electrode film of Ni will be briefly described. FIG. 1 is one example of an electrode structure of a thin film transistor having a semiconductor film of tellurium (Te) and source and drain electrodes of nickel (Ni). A stop layer 11 for stopping the effect of etching is formed on a glass substrate 10 and is a protective film for preventing the glass substrate 10 from being etched by the etching atmosphere used to form the gate electrode. A tantalum (Ta) film (only partially shown) is formed on the stop layer 11 by sputtering or vacuum deposition. The tantalum film is then formed into a gate electrode 12 by patterning, and part of the gate electrode 12 is subjected to anodic oxidation to form a gate insulating film 13. Then, a photoresist is used to form a mask, that can be lifted off according to a desired semiconductor pattern, and Te is vacuum-deposited thereon. A semiconductor film 14 of Te is formed by removing the photoresist. In the same manner, Ni is vacuum deposited to form a source electrode 15 and a drain electrode 16. Finally, aluminum oxide (Al.sub.2 O.sub.3) is vacuum deposited to form a protective film 17 which is adapted to protect the transistor from the environment. In this manner the thin film transistor element is manufactured.
If during formation of the source and drain electrodes 15 and 16 in the above-described manufacturing process, the Ni film is thin, then the formed source and drain electrodes 15A and 16A, as shown in FIG. 2(A), are discontinuous and therefore do not contact the semiconductor film 14. On the other hand, if the Ni film forming source and drain electrodes 15B and 16B is made thick enough to overcome the above-described difficulty, the Ni film may come off the substrate 10 because the Ni film will not sufficiently adhere to the substrate 10. The simplest method of increasing the strength of adhesion of the Ni film is to increase the temperature of the substrate. However, since the vapor pressure of the Te forming the semiconductor film 14 is high, and the source 15 and drain 16 electrodes are formed by the lift-off method using the photoresist, it is not possible to increase the temperature of the substrate to higher than about 100.degree. C.
Thus, it is difficult to provide a thin film transistor having good reproducibility and stable characteristics using the above-described process.